1. Field of the Invention
The present invention relates to a mechanism for controlling a melt level in a single crystal pulling apparatus, a method for controlling a melt level in a single crystal pulling apparatus, a mechanism for adjusting a melt level in a single crystal pulling apparatus, and a method for adjusting a melt level while pulling a single crystal that make it easy to control the position of the melt surface prior to or during a crystal growth in a single crystal pulling furnace using the Czochralski method (hereinafter, this may be referred to as the CZ method) for manufacturing single crystals of materials for semiconductor devices.
This application claims priority from Japanese Patent Application No. 2005-213233 filed on Jul. 22, 2005, the content of which is incorporated herein by reference.
2. Background Art
Various methods exist for pulling single crystals of materials for semiconductor devices, and the Czochralski method is one of these. FIG. 1 shows an example of a single crystal pulling apparatus that employs the Czochralski method. In FIG. 1, the symbol 1 is a crucible. Raw material for semiconductors is contained in the crucible 1 in the form of a melt 2. The symbol 3 is a heater. The melt 2 inside the crucible 1 is kept in a molten state by this heater 3. The symbol 4 is a single crystal shown as it is being pulled. A seed crystal 4B fixed at the distal end of a wire 4A is placed in contact with the surface of the melt 2, and then the single crystal 4 is pulled. The symbol 5 is a pulling apparatus. The single crystal 4 is gradually pulled by the pulling apparatus 5 via the wire 4A and the seed crystal 4B. The symbol 6 is a rotation apparatus. The single crystal 4 is rotated together with the pulling apparatus 5 by the rotation apparatus 6. The symbol 7 is a camera. The melt 2 inside the crucible 1 is photographed by the camera 7. The symbol 8 is an image processing device. Image data photographed by the camera 7 is processed by the image processing device 8. The symbol 9 is a control unit. The pulling apparatus 5 and the rotation apparatus 6 and the like are controlled by the control unit 9. The symbol 10 is a drive motor. The crucible 1 is moved up and down by the drive motor 10 so that the surface of the melt 2 is moved up and down. The symbol 11 is a heat shield. This heat shield 11 is provided so as to surround the single crystal 4 and to also insulate and maintain the temperature of the single crystal 4. Note that a lifting mechanism includes the control unit 9 and the drive motor 10.
The CZ method is a process in which the single crystal 4 is pulled from the melt 2 in the crucible 1 by the pulling apparatus 5 while being rotated by the rotation apparatus 6. At this time, in order to obtain a silicon single crystal of good quality continuously, for each batch, it is necessary to control the position of the melt such that the surface position of the melt 2 is kept constant relative to the heater 3.
In order to control the position of the melt such that the position of the melt surface is kept to a constant position in the initial stages of the single crystal pulling, the following methods are conventionally used.
A method (Patent Document 1) in which a non-combustible rod made of quartz or the like that is attached to the distal end of a furnace internal structural object is placed and the position of the melt surface is verified by lifting a crucible such that a melt in the crucible is in contact with the non-combustible rod;
a method (Patent Document 2) in which the position of the melt surface is verified by making a seed crystal in contact with the melt surface; and
a method (Patent Document 3) in which the position of the melt surface is verified by comparing reference marks and reflection images thereof with pre-recorded images or by making a direct calculation from the image data.
In Patent Documents 1 and 2, errors in the length of the non-combustible rods and the seed crystals, as well as errors made when confirming the contact with the melt that are due to the movement of the melt and the like create problems, and it is difficult to accurately set the position of the surface in the initial stages.
In Patent Document 3, in the case in which the image data photographed by the camera is compared in an unmodified manner with the comparison image data, or in the case in which a distance between the reference mark and the melt is directly calculated from the image data, errors in the camera mounting position and the like also remain and the accuracy is poor.
It is also possible to apply a measuring method such as laser triangulation or the like; however, in this case, the measuring apparatus itself is expensive, and it is necessary to provide a dedicated window for measuring purpose in the furnace body which also causes high equipment expenses.
The present invention was conceived in view of the problems in these conventional methods, and it is an object thereof to provide a mechanism for controlling a melt level (position of a melt surface) in a single crystal pulling apparatus, a method for controlling a melt level in a single crystal pulling apparatus, a mechanism for adjusting a melt level in a single crystal pulling apparatus, and a method for adjusting a melt level while pulling a single crystal that make it possible using existing equipment to easily and accurately adjust the position of the melt surface to a set position.    (Patent Document 1) Japanese Patent No. 1676655    (Patent Document 2) Japanese Patent No. 2132013    (Patent Document 3) Japanese Patent Application, Publication No. H06-116083